Angioplasty is an effective medical procedure performed to expand constricted sections of blood vessels. In such a procedure, an angioplasty balloon or balloon catheter is navigated to the site of the constriction. The balloon is inflated after reaching the site, by way of fluid pressure injected into the balloon, to thereby expand its dimension. The expansion of the balloon exerts pressure on the vessel walls to thereby widen the vessel and alleviate constriction to blood flow.
Conventionally, such balloons are manufactured from a polymeric material and are molded in a blow molding procedure. More specifically, a cylinder or tube of polymeric material, known as a parison, is placed within a mold having an interior cavity in the desired shape of the balloon. The mold is then heated, with the heat of the mold being conducted to the parison, such that upon introduction of fluid pressure into the parison the polymeric material deforms into the shape of the mold cavity. The mold is then cooled to cause the polymeric material to harden into the shape of the mold.
Typically, the mold is provided in a clam shell design wherein each half of the mold includes half of the interior cavity forming the balloon. The mold can therefore be wrapped around the parison and be easily removed to facilitate production. The parison itself can be heated by immersing the entire mold within a hot water, oil, glycerin, or other fluid bath and allowing the mold and parison to be heated via conduction. One problem associated with such a process is that heating of the parison is less than optimal. Heating via conduction, by its very nature, is a relatively slow process. Moreover, the substantial time it takes to heat the parison in the central section having the widest distance between the mold and the parison, in comparison to the narrow space at both ends, lends itself toward a substantial heat flow axially along the parison at these end sections, which itself tends to heat portions of the polymeric material at which balloon deformation is not desired. Accordingly, such systems typically need to employ some sort of cooling mechanism, such as a cold air jet, to keep the areas of the parison outside of the mold cool. One problem stemming from such a system is that temperature control or distribution across the entire polymeric tube is difficult. For bigger balloon sizes, in which the gap between the polymeric tube and mold wall is too large to give sufficiently fast transfer of heat, small amounts of water are often injected inside the mold between the parison and the mold for better heat conduction. However, it will be clear that this material is obstructing the free expansion of the parison inside the mold.
Moreover, with such conventional systems, it is not possible to heat different axial sections of the polymeric tube to different temperatures. For example, this may be advantageous when it is desired to create different physical properties within the balloon itself such as multiple areas of varying diameter, wall thickness, or multiple areas consisting of different materials to be heated to different temperatures. In a particular example one can think of the following: the tapering of the balloon from the central balloon section towards the shaft causes the wall thickness in the cone to increase towards the shaft section. This material distribution causes the folded balloon to be thicker in these cone sections than within the central section. For reasons of minimizing the profile of the product to achieve better access into the vascular system, one wishes to reduce the amount of material within the cone section and one way would be to heat the cone sections of the balloon to a higher temperature within the molding process in order to thin these sections. This effect of thinning would be the result of the combination of the applied axial force and the lower viscosity of the cone sections compared to the central cooler section. Although a section of the mold can be kept above the fluid bath, and thus have the effect of producing a cooler section in the mold, due to the slow heating process a sharp temperature transition is not possible. It is also not possible to set the metal mold to a different temperature than that to which the polymeric tube is heated. The mold must therefore be cooled down before the balloon can be removed.
In the construction of medical devices in addition to balloons, such as stents, guidewires, vena ceva filters and filter wires, the time required to cure adhesives and polymer coatings and thus facilitate manufacture, is relatively extensive. It would therefore be advantageous if a method could be devised for accelerating the curing process and thus manufacturing time for such medical devices.
Extrusion of polymers, such as used for medical products in the angiography field, has many inherent problems. One problem is the reduction of the transition zone occurring between two polymers being extruded on an intermittent basis, due to the combination of large volumes in the extruder head as compared to the volume of the medical device, such as a catheter tube, being extruded. There are also the high extrusion pressures present in combination with the elasticity of the polymers, as well as the shear forces occurring along the extruder wall.
Other extrusion-related problems include the fact that large, expensive, and complicated machines are necessary in the extrusion process to heat polymers homogeneously by a combination of mixing by the rotating screw, generating high shear forces, and simultaneous heat conduction through the heated inner surfaces of the extruder elements. Also, the processing time of polymers inside an extruder barrel and head is quite long. Such an extended processing time can have a signification degradation effect on the polymers being used, and in turn on the physical properties of the extruded product or so-called extrudate.
There are problems present in the cooling of extruded polymer products, including the length of cooling bath required, the need to have blowers to dry off the extrudate after being cooled in a cooling bath, and the need to quickly cool the heated polymers in the extrudate to minimize the effects of extended heating of the polymer material being extruded.